The amount of information (transmission capacity) transmittable through one optical fiber reaches a limit because a wavelength bandwidth of an optical fiber amplifier has been substantially used up due to an increase in the number of wavelength channels and an increase in a modulation speed of an optical signal. Further, in order to increase the transmission capacity of the optical fiber, there is a need to devise a signal modulation format, crow a large number of optical signals into a limited frequency range, and enhance the use efficiency of the frequency range.
In the world of a radio communication, a multilevel modulation technology has enabled such a high-efficient transmission that the frequency use efficiency exceeds 10 from the 1960's. The multilevel modulation has been also desired in the optical fiber transmission, and frequently studied up to now.
For example, in R. A. Griffin, et al., “10 Gb/s Optical Differential Quadrature Phase Shift Key (DQPSK) Transmission using GaAs/AlGaAs Integration,” OFC2002, paper PD-FD6, 2002 (Non Patent Literature 1), quadrature phase shift keying (QPSK) that conducts quaternary phase shift keying has been reported. In N. Kikuchi, K. Mandai, K. Sekine and S. Sasaki, “First experimental demonstration of single-polarization 50-Gbit/s 32-level (QASK and 8-DPSK) incoherent optical multilevel transmission,” in Proc. Optical Fiber Communication Conf. (OFC/NFOEC), Anaheim, Calif., March 2007, PDP21. (Non Patent Literature 2), a 32-level amplitude and phase modulation of a quaternary amplitude modulation and an eight-level phase modulation are combined together has been reported.
FIGS. 1(A) to 1(D) are diagrams illustrating a complex phase plane used for the optical transmission, and signal constellations of various known modulation formats. Signal points of various optical multilevel signals (complex indication of optical field at identification time) are plotted on the complex phase plane (or complex plane, phaser plane, IQ plane).
FIG. 1(A) is an illustrative view of the signal points on the IQ plane, and the respective signal points can be indicated by complex Cartesian coordinates (IQ coordinates) or polar coordinates represented by an amplitude r(n) and a phase φ(n) illustrated in the figure.
FIG. 1(B) illustrates a quaternary phase shift keying (QPSK) that transmits 2-bit information (00, 01, 11, 10) by one symbol with the aid of four values (π/4, 3π/4, −3π/4, −π/4) as a phaser angle φ(n).
FIG. 1(C) illustrates a 16-value quadrature amplitude modulation (16QAM) widely used by radio. In the 16QAM, the signal points are arranged in a grid-like pattern, and information transmission of 4 bits per one symbol is enabled. In an example illustrated in the figure, two values of higher-order bits (10xx, 11xx, 01xx, 00xx) are expressed by Q-axial coordinates, and two values of lower-order bits (xx10, xx11, xx01, xx00) are expressed by I-axial coordinates. There has been known that the signal constellation has a high receiver sensitivity because a distance between the respective signal points can be increased, and there has been reported that the quadrature amplitude modulation of this type can be realized by the aid of a coherent optical receiver. For example, in J. Hongou, K. Kasai, M. Yoshida and M. Nakazawa, “1 Gsymbol/s, 64 QAM Coherent Optical Transmission over 150 km with a Spectral Efficiency of 3 Bit/s/Hz,” in Proc. Optical Fiber Communication Conf. (OFC/NFOFEC), Anaheim, Calif., March 2007, paper OMP3. (Non Patent Literature 3), there has been reported an experimental example of transmission and reception of a 64QAM signal. The coherent receiver is of a system using a local laser source arranged in the interior of a receiver for detecting the phaser angle of the optical signal.
Now, a description will be given of a coherent receiving system that is one of the conventional optical multilevel receivers, for example, a coherent optical field receiver that has been reported in M. G. Taylor, “Coherent detection method using DSP to demodulate signal and for subsequent equalization of propagation impairments,” paper We4.P.111, ECOC 2003, 2003 (Non-Patent Literature 4).
FIG. 2 is a configuration diagram of a conventional digital coherent optical multilevel transmission system using a polarization diversity coherent optical field receiver.
In an optical multilevel transmitter 100, an unmodulated laser beam output from a laser source 106 is input to a quadrature optical field modulator 107, and an output optical signal 109 that has been subjected to a given field modulation is output from an output optical fiber 108. An information signal to be transmitted is input to a digital information input terminal 101 as a parallel (for example, m bit width) binary high-speed digital electrical signal string. The signals are converted into a complex multilevel information signal 103 cohered every several bits by a complex multilevel signal generator circuit 102. The signal is a digital electric multilevel signal expressed by (i(n), q(n)) (n is a sample number) on a two-dimensional IQ plane, and a real part i and an imaginary part q thereof are output every time interval T (=symbol time). After those signals have been converted into high-speed analog signals by DA converters 104-1 and 104-2, the signals are amplified by driver circuits 105-1 and 105-2, and input to two modulation terminals I and Q of the quadrature optical field modulator 107. As a result, the output optical signal 109 becomes an optical field signal of a complex multilevel signal (i, q) having an in-phase component I and a quadrature-phase component Q of the optical field. An optical field of the optical amplitude and phase modulation signal is (i(n)+jq(n))exp(jω(n)), and ω(n) is an optical angular frequency of the laser source 106. In this example, the DA converters 104 are used for multilevel modulation. However, if the number of multilevel is small, for example, if quaternary phase shift keying is realized, two pairs of binary signals may be applied to the quadrature optical field modulator without using the DA modulator.
After the output optical signal 109 has been transmitted through an optical fiber transmission channel 122, and undergone transmission degradation due to chromatic dispersion of the optical fiber, the output optical signal 109 is input to a digital coherent optical receiver 120. An input optical signal 121 is split into four types of an in-phase component of a horizontal (S) polarization, a quadrature-phase component of the horizontal polarization, and the in-phase component and the quadrature-phase component of a vertical (P) polarization by a polarization-diversity optical 90-deg. hybrid circuit 113, which are input to balanced optical receivers 110-1, 110-2, 110-3, and 110-4, respectively. A local laser source 112 arranged within the receiver is used as a reference of the optical phase of the received light, and has the substantially same wavelength as that of the input optical signal 121. An output light of the local laser source 112 is connected to another input port of the polarization-diversity optical 90-deg. hybrid circuit 113, and distributed to the balanced optical receivers 110-1, 110-2, 110-3, and 110-4 as with the signal light. In the respective balanced optical receivers, the input signal lights interfere with the local light so as to be converted into electrical signals, and then subjected to time sampling and converted into digital signals by respective AD converters 111-1, 111-2, 111-3, and 111-4. Those digital signals are first input to chromatic dispersion compensator circuits 114-1 and 114-2 for each polarization component, then input to an adaptive equalizer circuit 115. After the digital signals have been subjected to compensation for modulation distortion, waveform distortion caused by the remaining chromatic dispersion, and change and polarization dispersion of a polarization state, the digital signals are input to a downstream phase estimation circuit 116. Two sets of multilevel signals from which phase fluctuation has been removed are input to a multilevel signal decision circuit 117 in which the multilevel signals are subjected to symbol decision processing, and then decoded to original bit strings.
The above multilevel transmission suffers from a serious problem such as a modulation distortion of the generated optical multilevel signal. FIG. 3 is an illustrative view of a problem to be solved by the present invention, and illustrates the modulation distortion and an appearance of the equalization of the modulation distortion in the conventional digital coherent optical multilevel transmission system. The multilevel signal output from the complex multilevel signal generator circuit 102 is an ideal multilevel signal described in digital information. For example, in an example of the quaternary phase shift keying, the complex signal constellation is represented as illustrated in FIG. 3(A), and an error and a distortion are not included at positions of the signal points at all. However, in a process where the multilevel signals are converted into the high-speed analog signals, and converted into the optical signals by the DA converters 104-1 and 104-2, the multilevel signals suffer from large waveform degradation. As its factors, for example, there are shortage of a modulation bandwidth in the DA converters 104, the driver circuits, and the quadrature optical field modulator 107, reflection of a high-frequency signal in connectors and the respective components arranged in the channels, and differences of timing among plural modulation signals during modulation. If there are those degradation factors, the optical field of the output signal 101 causes an error at the signal point position as illustrated in FIG. 3(B), which causes a large degradation of a code error rate of the received signal.
However, because the digital coherent optical receiver 120 according to this example can take the optical field of the optical signal within the receiver as it is, the internal adaptive equalizer circuit 115 can be used to compensate a part of the modulation distortion. FIG. 3(C) illustrates a signal constellation to be input to the adaptive equalizer circuit 115, which is substantially identical with that of the output optical signal 109 (in this example, for simplification, influences of the chromatic dispersion, the fluctuation of polarization, and the phase fluctuation are ignored). If the modulation distortion caused on a transmitter side is linear, and a channel response time is finite, this waveform distortion can be substantially completely equalized by the aid of a digital adaptive equalizer filter such as a multistage transversal filter as the adaptive equalizer circuit 115. As a result, as illustrated in FIG. 3(D), the modulation distortion can be substantially completely removed from the output signal point, thereby making it possible to prevent the deterioration of the transmission characteristic such as the code error rate.
On the other hand, FIG. 4 is a configuration diagram of a phase pre-integration type optical multilevel signal transmission system using direct optical detection that has been proposed by the present inventors in advance. This system easily realizes the optical multilevel transmission by the aid of optical delay detection without using the coherent detection and the local laser source, and a detail of the system is disclosed in Patent Literature 1: WO2009/060920.
A basic configuration (the laser source 106, the quadrature optical field modulator 107, the complex multilevel signal generator circuit 102, the DA converters 104, the driver circuits 105, etc.) of a phase pre-integration type optical field transmitter 123 is roughly identical with the optical multilevel transmitter 100 in FIG. 2. However, because the direct optical detection is used, a part of the internal signal processing is different. In this example, the multilevel signals output from the complex multilevel signal generator circuit 102 are input to a phase pre-integration circuit 126, and converted into phase pre-integration complex multilevel information signals in which only phase components are digitally integrated at a time interval T within the phase pre-integration circuit 126. When the complex multilevel information signals (i, q) to be input are converted into the polar coordinates on the complex plane, the polar coordinates can be described, for example, as Ei(n)=i(n)+jq(n)=r(n)exp(jφ(n)) (j is an imaginary unit). In this expression, n is a symbol number of the digital signal, r(n) is a symbol amplitude of the digital signal, and φ(n) is a phaser angle. In this case, the phase pre-integration signals to be output can be also described as Eo(n)=i′(n)+jq′(n)=r(n)exp(jθ(n))=r(n)exp(jΣφ(n)) in the polar coordinates. In this expression, θ(n) is a phaser angle of the output signal, Σφ(n) is a value obtained by accumulating past phaser angles φ(1) . . . φ(n). After the output signals have been again converted into the Cartesian coordinates, the output signals are input to a complex up-sampling circuit 124, and sampling points are complemented so that a sampling speed becomes 2 samples/symbol or more. As a result, the Nyguist theorem is satisfied to enable complete field equalization processing. Thereafter, an inverse function of the degradation caused by the optical fiber transmission channel 122 is applied to the signals, and converted into complex signals i″ and q″ by a preequalizer circuit 125. After those signals have been converted into the high-speed analog signals by the DA converters 104-1 and 104-2 as with the optical multilevel transmitter in FIG. 2, the signals are converted into optical field signals (i″(n)+jq″(n))exp(jω(n)) by the quadrature optical field modulator 107, and output.
After the output optical signal 109 has been transmitted through the optical fiber transmission channel 122, and undergone the transmission degradation due to the chromatic dispersion of the optical fiber, the output optical signal 109 is input to an incoherent optical multilevel receiver 130 as the input optical signal 121. An influence of the chromatic dispersion in the optical fiber transmission channel is mutually canceled by the inverse function applied by the preequalizer circuit 125 in advance, and therefore the input optical signal 121 is equivalent to the output signal of the phase pre-integration circuit 126.
The input optical signal 121 is split to three optical signal channels by an optical splitter 132, and input to a first optical delay detector 133-1, a second optical delay detector 133-2, and an optical intensity receiver 135. The first optical delay detector 133-1 is set so that a delay time difference Td between two internal optical channels becomes substantially equal to the symbol time T of the received optical multilevel information signal, and an optical phase difference between both the channels becomes 0. Also, the second optical delay detector 133-2 is set so that the delay time difference Td between those two internal optical channels becomes substantially equal to T, and the optical phase difference between both the channels becomes π/2. The output optical signals of the first and second optical delay detectors 133-1 and 133-2 are converted into electrical signals by balanced optical receivers 134-1 and 134-2, and thereafter converted into digital signals dI(n) and dQ(n) by AD converters 136-1 and 136-2, respectively. Also, an output electrical signal of the optical intensity receiver 135 is also converted into a digital signal P(n) by an AD converter 136-3.
Then, after the digital signals dI(n) and dQ(n) have been input to the adaptive equalizer circuit 115-1, and a part of the waveform distortion has been removed from the digital signals, the digital signals dI(n) and dQ(n) are input to an inverse tangential operation circuit 137. In this circuit, an inverse tangential operation of second argument having dI(n) as an X component and dQ(n) as a Y component is conducted to calculate the phaser angle. When the optical field of the input optical signal 121 is described as r(n)exp(jθ(n)), dI and dQ are written as dI=r(n)r(n−1)cos(Δθ(n)) and dQ=r(n)sin(Δθ(n)) from the principle of the optical delay detection. In this example, Δθ(n) is a phase difference (θ(n)−θ(n−1)) of a received n-th optical field symbol from a prior symbol. Because dI and dQ are a sine component and a cosine component of Δθ(n), respectively, the inverse tangential operation circuit 137 can conduct the inverse tangential (inverse Tan) operation of four quadrants to calculate Δθ(n).
In this configuration, because the phase pre-integration is conducted on the transmitter side, the phaser angle of the received optical field signal is θ(n)=Σφ(n). Hence, the output signal of the inverse tangential operation circuit 137 becomes Δθ(n)=Zφ(n)−Σφ(n−1)=φ(n), and a phase component φ(t) of the original complex multilevel information signal 103 can be extracted.
On the other hand, after a part of the waveform distortion has been removed from the output signal P of the optical intensity detector by the adaptive equalizer circuit 115-2, the output signal P is input to a square root circuit 138 so as to obtain an original field amplitude r(n)=sqrt(P(n)) as an output. Hence, the amplitude component r(n) and the phase component p (n) thus obtained are input to a Cartesian coordinate converter circuit 139 so as to reproduce an original digital electric multilevel signal (I, Q)=r(n)exp(Δθ(n)). The original digital electric multilevel signal is input to the multilevel signal decision circuit 117 to reproduce the information signal.